STUDIES IN MAMMALIAN EMBRYOLOGY. 285 
would be found, there being evidently considerable variation 
in the time of conception and of birth. 
At present I dispose of about five hundred embryos belong- 
ing to very different stages. A great many of these are early 
stages, and give a sufficient basis for the careful study of the 
principal processes both of organogeny and of placentation. I 
feel justified in publishing certain results I have obtained up 
to the present date, more especially concerning the earliest 
didermic stages and the phenomena of placentation. 
It will be seen that in very many points there is a close resem- 
blance, in others, however, an unexpected divergence between 
the hedgehog and the mole, as far as we are acquainted with its 
development through Heape’s valuable papers (‘ Quart. Journ. of 
Mier. Sci.,’ 1883, 1886). I may add that the significance of such 
divergent phenomena of development occurring among different 
genera of Insectivora has certainly not been lessened by the 
fact which I have since been able to observe, viz. that in a 
third genus of Insectivora, the shrew, the early stages of the 
blastocyst and the phenomena of placentation again follow a 
very different course. These points will, however, not be 
touched upon in this paper, which has already grown to a 
bulk (especially with reference to figures and plates) that 
must needs bring a frown to the editor’s brow, to whose kind 
liberality I wish to tender special thanks. Also to my friends 
Drs. de Graaf and Vosmaer, for their valuable aid in the illus- 
trative part, and to my artist, Mr. J. G. de Groot. Although 
the phenomena of placentation and of the development of the 
foetal membranes will occupy by far the greater portion of 
this memoir, I must needs commence with a description of the 
very early stages of the blastocyst, without which those phe- 
nomena could not be understood. The development of the 
mesoblast and the general organogeny of the hedgehog will be 
treated of in a later publication. 
